Pressurized-water reactors include a generally cylindrical vessel which contains the reactor core and which is arranged with its axis vertical in a vessel pit emerging via its upper end into a pool.
The vessel includes a closing head at its upper portion and a convex bottom head at its lower portion, the later being penetrated by a plurality of substantially vertical tubes called vessel bottom-head penetrations.
These vessel bottom-head penetrations include an end projecting inside the reactor vessel and an opposite end which projects beneath the convex bottom head and is connected to a flexible measurement conduit enabling the bottom head of the vessel to be joined to an instrumentation room arranged in the structure of the reactor building, generally adjacent position to the vessel pit.
Each of the measurement conduits and the corresponding bottom-head penetration provides the passage for a thimble in which moves a measurement probe fixed to the end of a very long flexible element and its insertion into the vessel and the core in order to carry out measurements, for example neutron flux or temperature measurements, inside the core while the reactor is operating.
The tubes for penetrating the vessel bottom head are generally made of nickel alloy and are welded to the convex bottom head of the vessel which is generally made of high-strength steel.
The inner surface of the vessel bottom-head penetrations is in contact, during the operation of the reactor, with the primary reactor coolant constituted by pressurized water containing various additives.
A certain tendency towards cracking of the inner surface of the vessel bottom-head penetrations has been observed, in particular in the vicinity of the zone in which the penetration tube is welded to the vessel bottom head.
It may be necessary to repair the inner surface of the penetrations after the reactor has been operating for a certain time, and it is advantageous, in any case, to treat this inner surface preventively in order to prevent or to delay the cracking of the inner wall of the penetrations while the rector is operating.
In the case of tubular elements, which can be subjected to stress corrosion and to cracking in contact with the primary coolant of a reactor, for example in the case of the tubes of the steam generators of pressurized-water nuclear reactors, repair methods or procedures for preventive treatment of the inner wall of the tubular elements have been laid down which enable their lifetime to be extended and the safety of the nuclear reactor to be improved.
It has thus been anticipated, in the case of the tubes of steam generators which include a transition zone between a deformed portion and a non-deformed portion, in the vicinity of the upper face of the tube plate into which these tubes are fixed, to treat their inner wall, in the transition zone, for example by mechanical expansion, thermal stress-relieving or peening by means of small balls (shot peening).
In the case where cracks have appeared, it is also possible to carry out a repair by sleeving the tube in the transition zone.
Processes and devices have also been proposed, in applicant's FR-A-2,565,323, FR-A-2,585,817, FR-A-2,615,207 and FR-A-2,652,191, for depositing metal by electrolysis onto the internal surface of the steam-generator tubes in the transition zone. This treatment makes it possible to prevent contact between the primary reactor coolant and the transition zone which is susceptible to stress corrosion and to cracking.
The metal deposited by electrolysis is generally pure nickel, the steam-generator tubes being made of a nickel-based alloy containing a proportion of nickel of the order of 75%.
However, a process for repairing and protecting and from cracking, the inner wall of the vessel bottom-head penetrations of a nuclear reactor, after the reactor has been operating for a certain time, was not known to date.